A conventional Stirling refrigerator is designed, for example, to cool infrared sensors and detectors in thermal imagers operating at a temperature of 60-140 K. Such conventional refrigerator generally comprises a compressor 10, and a cold finger 20 as shown by FIG. 1. The compressor 10 and the cold finger 20 are constructed as separate components connected together through a conduit 30. This split configuration provides maximum flexibility in system design and isolates the detector from the compressor-induced vibrations.
The compressor 10 includes a cylinder fit 12 within a compressor housing 11. In the example of FIG. 1, two pistons 13 are mounted for reciprocal action within the cylinder 12. The use of dual-opposed pistons driven by linear motors minimises compressor vibration and acoustic noise. A helical suspension spring 14 is horizontally disposed between each piston 13 and the compressor housing 11. A compression chamber 15 having a variable volume is defined in the cylinder 12 between the two pistons 13. The pistons 13 are driven by linear motor using coil placed inside the working gas. The coil is attached to the piston 13. A permanent magnet 18 is connected to the compressor housing 11.
The cold finger 20 includes a cylinder 23 within which a displacer 24 is reciprocal. A regenerator or regenerative heat exchanger is integrated in the displacer 24. A helical displacer spring 25 is disposed under the displacer 24.
The gas pressure fluctuations in the compression chamber 15 acts on the spring load displacer 25. This gas spring system is tuned to provide a good practical approximation to the ideal phase relationship between the displacer 24 and the pistons 13. Refrigeration occurs around the top 21 of the cold finger 20, which contains an expansion space 27. The displacer 24 separates this space 27 from a compression space consisting of the space 15 between the two pistons 13, the space in the split tube 30 and the space below the warmer end of the displacer 24.
The phase difference between the movement of the displacer and the movement of the piston is designed in such a way that compression occur when the expansion space is small and expansion of the gas occurs when the expansion space is large. In this way, more gas in the expansion space is being expanded and cooled than it is compressed (and heated). Thus resulting in a net cooling effect generated at the top of the cold finger in the expansion space.
In the start of the first phase of the Stirling cycle, the gas is in The compression chamber 15 at ambient temperature and the displacer 24 is in the top 21 of the cold finger 20. The pistons 13 are driven inwards, compressing the gas. This process is nearly isothermal; the heat output being dissipated via heat sinks around the compressor 10 and the base of the cold finger 10. To reduce the required heatsink capacity of the warm end of the cold finger 20, the cooler is equipped with a Heatstop™ 40 in the cold finger 20 or transfer line 30.